The present invention relates to dot matrix printing apparatus and more particularly to an arrangement for reducing the stresses and hence resultant breakage of printing styli employed in such apparatus.
In the so-called dot matrix printer, visual characters are formed by groups of dots imprinted on paper. Each dot is produced by driving an impact end of an elongated printing element against the platen for the paper and an ink ribbon disposed between the printing element and the platen. Commonly a plurality of printing elements are arranged in parallel to form a line transverse to the direction of travel of the paper. As the paper and printing elements are driven relative to one another, the dots are produced in successive parallel rows, one row at a time, by selectively actuating drivers which cause the printing elements to extend and retract according to the characters to be printed. Typical of this class of printers is the apparatus disclosed in French Pat. No. 2,094,311 entitled "Improvements in Printing Heads for Printing Machines", allowed Jan. 10, 1972 in the name of LogAbax SA, U.S. Pat. No. 3,829,908 entitled "Mosaic Print Head" issued in the name of Winfred Schneider, U.S. Pat. No. 3,842,955 entitled "Dot Printer" issued in the name of Kyubachiro Iwasaki dated Oct. 22, 1974.
In a dot matrix printer provision must be made for selectively actuating the printing element to imprint the dots on the paper. The actuating mechanisms are of necessity located in the vicinity of their respective elements. Thus, space occupied by the actuating mechanisms places a limit on the distance between the parallel printing elements in a line and accordingly the size of the characters. Various schemes have been resorted to for providing space for the element actuators while achieving suitable driving forces. Because of limited manufacturing tolerances, particularly in large scale production, difficulties occur in achieving proper printer operation uniformly among individually produced print heads. Efforts to control tolerances have lead to costly and unreliable print heads. In particular, it is desirable to control the printing forces developed by each of the printing elements. It is impractical particularly where large scale production is involved to customize each print head so that the desirable operating characteristics are achieved. It is desirable, as in the present instance, to maintain production tolerances within a practical level that results in both reliable and relatively inexpensive units while providing for some degree of individual adjustment. This is particularly true in the case of print heads employing driven printing elements. It is necessary to avoid the situation where the spring and other forces opposing the magnetic forces are so large that the magnet is unable to drive the spring into the record medium to affect printing or the forces are so relatively weak that the electromagnet drives the printing elements into the record medium with such force as to destroy the needle or injure the record medium.
Accordingly, it is one object of this invention to provide an improved print head for dot matrix printing.
It is another object of this invention to provide an individual adjustment for each print head to balance the forces on the printing element during the extending and retracting position to achieve optimum printing operation.
Another object of this invention is to provide an improved leaf spring arrangement for controlling the biasing force on the armatures used in driving the printing elements, particularly during their retracted position.
It is another object of this invention to control the degree of biasing of the armature with respect to a stop position by adjustably deflecting biasing leaf spring from a predetermined direction.
It is another object of this invention to provide improved means for adjusting a solenoid spring preload of each of the several printing elements or styli in a matrix printer.
The matters discussed above as well as further objects and features of the present invention will be more clearly understood and appreciated following a consideration of the accompanying drawings.